The cheapest in the range is the duo-tec 24 combi erp which typically costs £680, and the most expensive is the ecoblue 33 combi erp at £1,735. Tried the conventional methods of unblocking a drain or waste pipe? now here’s a natural method you could try too. Sulfuric acid + sodium hydroxide (base) → sodium sulfate (salt) + water. The county council is not responsible for maintaining roadside drains.